1. Field of the Invention
The present invention relates to the field of particle sorters and, in particular, to particle sorting by flow cytometry.
2. Description of Related Art
Flow-type particle sorting systems, such as sorting flow cytometers, are used to sort particles in a fluid sample based on at least one measured characteristic of the particles. In a flow-type particle sorting system, particles, such as molecules, analyte-bound beads, or individual cells, in a fluid suspension are passed in a stream by a detection region in which particle sensing means senses particles contained in the stream of the type to be sorted. The fluid suspension is jetted from a nozzle and vibrated to break the stream into uniform discrete drops. The signal, or signals, from the particle sensing means are processed to provide a signal output when a particle of the type to be sorted is sensed that triggers assorting mechanism to separate the drop containing the particle from the stream of drops. A number of methods of sorting particles are known in the art, including the use of moving droplet capture tubes (as described in U.S. Pat. No. 5,030,002) and electrostatic sorting. In electrostatic sorting, drop charging means are connected to the stream to charge drops containing a particle of the type to be sorted with an electrical charge as it breaks off from the jet stream. The stream of drops is passed through a transverse electrostatic field established by a pair of oppositely charged deflection plates. Uncharged drops are not deflected passing through the electrostatic field and are collected by a central receptacle. Charged drops containing a particle of the type to be sorted are deflected in a direction and amount related to the polarity and magnitude of the drop charge and are collected in a separate collection receptacle.
Particle sensing typically is carried out by passing the fluid stream by a detection region in which the particles are exposed to an excitation light, typically from one or more lasers, and the light scattering and fluorescence properties of the particles are measured. Particles or components thereof can be labeled with fluorescent dyes to facilitate detection, and a multiplicity of different particles or components may be simultaneously detected by using spectrally distinct fluorescent dyes to label the different particles or components. Typically, detection is carried out using a multiplicity of photodetectors to facilitate the independent measurement of the fluorescence of each distinct dye.
The transverse electrostatic field typically is established by a pair of oppositely charged deflection plates flanking the drop stream. The deflection plates are connected to opposite-polarity dc potential sources of, for example, from ±1000 to ±5000 volts. Positively charged drops passing between the deflection plates are deflected towards the negatively charge deflection plate, and negatively charged drops are deflected towards the positively charged deflection plate. The position and lengths of the deflection plates are such that deflected drops do not collide with a plate, but are deflected sufficiently to be collected by a separate collection receptacle held below plates.
Sorting flow cytometers similar to the type described above are described, for example, in U.S. Pat. Nos. 3,826,364; 4,667,830; 5,483,469; 5,700,692; 6,372,506; 6,809,804; 6,880,414; and 7,201,875; the entire contents of each patent being incorporated by reference herein. Flow sorters are described in “Flow Sorters for Biological Cells” by Tore Lindmo, Donald C. Peters, and Richard G. Sweet, Flow Cytometry and Sorting, 2d ed. (New York: Wiley-Liss, Inc., 1990), pages 145-169, and in Shapiro, 2003, Practical Flow Cytometry (John Wiley and Sons, Inc. Hoboken, N.J.), both incorporated herein by reference. Commercially available sorting flow cytometers include the BD FACSVantage™ and BD FACSAria™ sorting flow cytometers manufactured by BD Biosciences (San Jose, Calif.).
One problem that arises in the use of flow-type particle sorters results from the undesired formation during droplet formation of aerosols that can remain suspended in the air in the sort chamber. The aerosol particles can settle on the deflection plates, causing wetting of the deflection plates. Because of the high voltages applied to the plates, wetting of the plates can result in arcing and loss of deflection plate charge, thus interfering with the correct functioning of the sorter.